Multirange coupling circuit



June 28, 1938. w. D.4 LOUGHLIN vET AL' 2,122,257

MULTIRANGE COUPLING CIRCUIT Original Filed Sept. 4, 1950 INVEN'BgR WILLIAM D.L GHLIN BYCHRISTOP R J.FRANK5 /d ATTORNEY Patented June 28, 1938 UNITED STATES MULTIRANGE COUPLING CIRCUIT William D. Loughlin,

Mountain Lakes, and

Christopher J. Franks, Denvillc, N. .1., assignors to Radio Corporation of America, a corporation of Delaware Original application September 4, 1930, Serial No. 479,728. Divided and this application April 9, 1934, Serial No. 719,626

26 Claims.

This invention relates to multi-range coupling circuits and particularly to tuned circuits adapted to maintain efficient operation of Vacuum tube amplifiers when tuned over a plurality of. ranges '5 of signal frequencies, and this application is led as a divisional of application Serial No. 479,728, filed on September 4, 1930, and now issued as Patent No. 1,978,212, dated Oct. 23, 1934.

To obtain the desired sensitivity, selectivity and gain characteristics in a vacuum tube amplier that is to be operated over an extended range of frequencies it has been customary to employ a series of interchangeable inductances or capacities. Tapped impedances and switches have been considered unsatisfactory due to the Wide variations in the performance vof the .amplier when operated at different frequency ranges. While better operating characteristics may be obtained with plug-in reactances, the changing of the elements renders this method .unsuitable for many purposes, such as for example, in broadcast receivers employing a plurality of tuned amplifier stages.

An object of the present invention is to provide tuned coupling circuits including switches for changing the frequency band, which circuits eX- hibit satisfactory transmission characteristics for all frequency bands over which they may be tuned. A further object is to provide coupling circuits of the type stated which are particularly adapted for use as interstage couplings in cascaded amplifiers.

These and other objects Will be apparent'from the following specification, When taken with the accompanying drawing, in which Fig. 1 is a circuit diagram of a portion of a radio receiver which includes coupling circuits constructed in accordance with this invention.

Fig. 2 is a side elevation of one physical embodiment of the invention, and

Fig. 3 is a side elevation, with parts shown in section, taken at right angles to the Fig. 2 elevation.

In the drawing, the reference numerals I, 2 and 3 identify three vacuum tubes which are arranged in cascade by double range tuning circuits embodying the invention.

'The input circuit for the rst tube I takes the form of two substantially identical tuned circuits, each circuit including a high frequency inductance 4, a loading inductance 5 and a tuning condenser E. The tuned circuits are coupled by a common shunt impedance which preferably takes the form of a parallel circuit comprising a capacity 'I and an inductance 8. A series cou-55 pling capacity 9 is preferably provided between the high potential ends of the high vfrequency inductances 4, and this coupling may, and lpreferably is, obtained by a proper choice of the incif dental capacitive coupling between the tuned cir-"e510 cuits. The junction of the tuning condensersfis grounded, and the high potential terminals of the inductances l are connected to the' antenna'or other collecting structure IEB through a smallfcoupling condenser II, and to the control 4grid 'Gfii15 of tube I, respectively. *I

The inductances 4 and 5 of eachcircuit `are shielded from each other by mounting thesame in separate shields, or Within` a single shield by arranging the coils at right angles to reduceazo the magnetic coupling substantially to Zero. Switches I2 are provided for short circuiting the loading inductances 5 when it is desiredlto receive signals within the higher range of frequencies. l ff The tuned input circuit for the secondr tubel 2 includes a high frequency inductance 541,r a loading inductance 5 and tuning condenser 6 `which elements may be, and preferably are, identical with the corresponding elements of one oftheiw'go tuned circuits associated with the tube I.' Due to the fact that this tuned circuit is energized by the alternating Voltage established in the plate circuit of tube I, it Will be apparent/to"V one familiar With the operation of Vacuum j tube1f35 amplifiers that the customary methodsof coupling to the plate circuit will not give satisfactory operation over both ranges of frequencies. When screen grid tubes of the tetrode or pentode type are employed, and theinven-:fo tion is Well adapted for use with suoli tubes, it is impractical to couple the plate of tube I to the control grid of tube 2 through a blocking condenser and to supply the direct current potential to the plate P through a choke or resistance. :1;45

The interstage coupling to the double range tuned circuit includes separate couplings to the high frequency inductance ll and tothe loading inductance 5. As shown in Fig. 1, the plate circuit of tube I includesa coil I3 which iscoupledygo to the high frequency inductance ll of the tuned circuit. The direct current circuit to plate P is completed from the low potential terminal of coil I3 to the plate supply (not shown) through a radio frequency choke I4, and the alternating current circuit is completed to ground through the blocking condenser I5 and a portion 5a of the loading inductance 5. When the switch I2 is opened for reception of signals in the lower range of frequencies this connection of the plate circuit to an intermediate point in the loading inductance 5 results in an autotransformer action that provides the desired degree of coupling between the plate circuit and the loading inductance. This coupling is, of course, in such sense as to assist the magnetic coupling between coil I3 and the high frequency inductance 4.

A switch I6 is provided for shortcircuiting the portion 5a of inductance 5 when switch I2 is closed for reception in the higher frequency band. The resistor il is connected in shunt with the inductance 5 so that it acts as a losser resistance when the set is operated in the low frequency range. The grid bias path is through coils 5 and 4. The resistance Il introduces a broadening effect in the low frequency range of the receiver, and is short circuited when the switches I6 and I2 are closed for operating the receiver in the high frequency range. As shown diagrammatically, the coil I3 takes the form of a spiral or pancake winding to reduce the capacitive coupling between coil I3 and inductance 4 to a minimum. This feature is quite important since it would be difficult or impossible to employ a single condenser 6 for tuning over both ranges if the capacity between inductance 4 and the ground was relatively large. The elimination of the coupling between the plate circuit and the loading inductance 5 is essential if a satisfactory gain characteristic is to be obtained in the high frequency range. Except for the provision of the switch I 6, the leakage inductance in the loading inductance section of the tuned circuit would resonate with the inherent capacity of tube I within the high frequency range, thus destroying both gain and selectivity.

When high selectivity is desired, the coupling between two of the audions may take the form of the two coupled tuned circuits that are above described as comprising the coupling system between the antenna I6 and the first amplifier I. Such a coupling system is illustrated in Fig. 1 in connection with the audions 2 and 3. Each of the tuned circuits and the couplings between the same may be, and preferably are, substantially identical with corresponding elements of the input system of the first audion. The coupling between the plate circuit of tube 2 and the first tuned circuit conforms, however, to the /fcoupling between the plate of tube I and the tuned input circuit of the tube 2. Detailed description of the circuit elements is believed to be unnecessary as like elements are identified by the same reference numerals as those identifying similar elements of the input systems of tubes I and 2.

All of the tuning condensers 6 are connected in the usual manner by an appropriate mechanical connection I8 to insure the simultaneous tuning to resonance of all of the circuits. In practice, the condensers 6 preferably comprise the individual units of a gang condenser.

The -coupling impedance between the tuned circuits of each pair is essentially capacitive, but the inductance 8 is preferably included in shunt with the coupling capacity 'l' for two reasons. It provides a direct current path between ground and the control grid G1 of the next tube, thereby permitting the application of an appropriate bias voltage on the grid. The inductance 8 has the further function of improving the fidelity of reception at the extreme low frequencies. By so choosing the Value of inductance 6 that it resonates with capacity 'l at the low frequency end of the low frequency band, the impedance presented by the coupling unit increases as the frequency decreases and thereby increases the coupling between the circuits. This results in a broadening of the resonance curve for the lowest frequencies and thus reduces the cutting of the sidebands in this portion of the frequency spectrum.

One structural assembly of the coupling circuits which, in practice, has been found to be particularly eicient is illustrated in Figs. 2` and 3.

The inductance 4 has the form of a single layer winding upon tube I9 of insulating material that is supported, by brackets 20, upon one end and at right angles to a Second tube 2! upon which the inductanceA 5 is wound. To reduce the size and distributed capacity of inductance 5, this winding is preferably bank-wound in a plurality of sections, as is shown in Fig. 3. The coupling inductance I3 is wound in a narrow slot at the edge of a disc 22 that is mounted within the tube I9, and centrally of the Winding ll, by suitable means, such as drops 23 of cement.

The coupling condenser I5 and high frequency choke I4 are located within the tube 2l and, be, ,f

ing of relatively light weight may be supported solely by the connecting leads. The axis of the choke I4 is at right angles to the axis of winding 5 to reduce the magnetic coupling substantially to zero.

As will be apparent from Fig. 3, this construc-v l 24 on tube I 9, and the opposite end of winding' 4 is connected by a lead 25 to the adjacent end of the inductance 5. 'Ihe tube i9 carries a second terminal 26 that is connected to one end of the coupling inductance I3, the opposite end of the inductance being connected, by lead 21, to that terminal of the coupling condenser I5 to which the choke i4 is connected. The opposite condenser terminal is connected, by lead 28, to a terminal 29 that is carried by the lower end of the tube 2l. The intermediate tap 3i), on winding 5, is connected to the lead 28, as shown in Fig. 3, or it may be run directly to the terminal 26. The lower ends of the winding 5 and choke I4 are connected to terminals 3l and 32, respectively, that are mounted on tube 2I bled unit then consists of only the windings 4 andy 5, and this form is used for each coupled circuit of the input system between antenna I6 and tube I and for the second of the two coupled circuits employed as an interstage coupling between tubes 2 and 3.

The following data with reference to one particular assembly designed for use at a high frequency range of from 550 to 1500 kilocycles and a low frequency range of from 150 to 375 kilocycles, is given `merely as an example of circuit constants which have been convenient forfone embodiment of the invention.

Inductance 4:100 turns of `#30 (American wire gauge) enamel wire close wound in a single layer on a 1.25 inch tube.

While the invention has been described as applied to a coupling system adapted to operate eiciently over two frequency ranges, it will be apparent that the general method of design applies equally well when three or more ranges are to be covered.

We claim:

1. The combination with a tuned circuit comprising, in series two inductances and a tuning condenser, one terminal of said condenser being grounded, of a primary circuit coupled to Veach of said inductances for transferring energy to said tuned circuit, said primary circuit including a coil coupled magnetically to the inductance adjacent the non-grounded terminal of said condenser, and the coupling to said second inductance comprising a connection from-said coil to v.ground through a portion of said second inductance, and means for shortcircuiting onel of said inductances and for eliminating the coupling thereto of said primary circuit.

2. The combination with a tuned circuit comprising, in series two inductances and a tuning condenser, one terminal of said condenser being grounded, of a primary circuit coupled to each of said inductances for transferring energy to said tuned circuit, said primary circuit including a coil coupled magnetically to the inductance adjacent the non-grounded terminal of said condenser, and the coupling to said second inductance comprising a connection from said coilto ground through a portion of said second inductance such that the last mentioned primary circuit coupling assists the magnetic coupling between the primary coil and the first inductance.

3. The combination with a tuned circuit comprising, in series two inductances and a tuning condenser, one terminal of said condenser being grounded, of a primary circuit coupled to each of lsaid inductances for transferring energy to said tuned circuit, said primary circuit including a coil coupled magnetically to the inductance adjacent the non-grounded terminal of said condenser, and the coupling to said second inductance comprising a connection from said coilto ground through a portion of said second inductance such that the last mentioned primary circuit coupling assists the magnetic coupling between the primary coil and the lirst inductance, and

means for shortcircuiting one of said inductances and for eliminating the coupling thereto of said primary circuit.

4. The combination with a resonant circuit including, in series, a high frequency coil, a loading coil and a condenser, of a primary circuit for' transferring energy to said resonant circuit, said primary circuit including a portion of said loading coil and also a coil coupled magnetically to said high frequency coil, and means for shortcircuiting said loading coil and for shortcircuiting the portion thereof which is common to said primary circuit.

5. In a tuned amplifier, a tuned input circuit for a vacuum tube comprising, in series, a high frequency inductance, a loading inductance and a tuning condenser, a primary circuit comprising a portion of said loading inductance and a coil coupled to said high frequency inductance, and switching means for shortcircuiting said loading coil and the portion thereof which is common to said primary circuit.

6. In a double-range, tuned radio frequency coupling system, a secondary circuit including, in series, a tuning condenser and a pair of 1nductances having substantially Zero electromagnetic coupling, a primary circuit including an inductance magnetically coupled to one of said pair of inductances and a connection to ground through a portion of the other inductance of said pair, and switching means for shortcircuiting the second inductance of said pair and that portion of said second inductance which is included in said primary circuit.

'7. In a double-range coupling system, the combination with an inductance and tuning con-2.1

denser adapted to resonate therewith over one band of frequencies, and a second inductance in series with said first inductance and tuning condenser, of a primary circuit including a coil coupled to said first inductance and a low impedance capacity connection to one intermediate point in said second inductance, and switching means for shortcircuiting each of the sections into which said inductance is divided by lsaid intermediate point.

8. The combination with a primary circuit including spaced inductances with substantially zero magnetic coupling therebetween, ofa secondary circuit including, in series, a tuning condenser and two inductances having substantially Zero electromagnetic coupling therebetween, one inductance of said primary circuit being coupled to one inductance of said secondary circuit and the other inductance of said primary circuit being coupled to the other inductance of said secondary circuit, yand means for shortcircuiting one of said secondary inductances and the primary inductance coupled thereto.

9. In a tuned amplifier, two vacuum tubes, means for impressing a signal voltage upon. the input circuit of the rst tube, a plate circuit for said first tube including a coupling coil adjacent said plate and a second inductance in series with said coil but having substantially Zero magnetic coupling therewith, and a tuned input circuit for said second tube, said tuned input circuit including in series with said plate circuit inductance the following circuit elements; la tuning condenser, a high frequency inductance coupled magnetically to said coil, a loading inductance coupled'magnetically to said plate inductance and having substantially zero magnetic coupling with 'said'high frequency inductance, and

switching means for shortcircuiting said platev circuit inductance and for shortcircuiting said loading inductance.

l0. A tuned interstage coupling system of the type including two tuned circuits in series between the adjacent. tubes, said coupling comprising two substantially identical tuned circuits each having a tuning condenser and the common terminals of said condensers being grounded, a shunt coupling impedance connected to ground and to the joined terminals of loading inductances individual to said tuned circuits, a high frequency inductance completing each of said tuned circuits, a connection to the control grid of the second tube from the junction of the tuning condenser and high frequency inductance of said second tuned circuit, the plate circuit of the first tube including a coil coupled magnetically to the high frequency inductance of said first tuned circuit, the low potential terminal of said coil being connected through a'blocking condenser to an intermediate point in the loading inductance of said rst tuned, circuit, and means for shortcircuiting the loading inductance of said rst tuned circuit and the portion thereof which is common to the plate circuit of said first tube.

11. A tuned interstage couplingsystem of the type includingr two tuned circuits in series between the adjacent tubes, said coupling comprising two substantially identical tuned circuits each having a tuning condenser and the common terminals of said' condensers being grounded, a shunt coupling impedance connected to ground and to the joined terminals of vloading inductances individual to said tuned circuits, a high frequency inductance completing each of said tuned circuits, a connection to the control grid of the second tube from the junction of the tuning condenser and high frequency inductance of said second tuned circuit, the plate circuit of the first tube including a coil coupled magnetically to the high frequency inductance of said first tuned circuit, the low potential terminal of said coil being connected through a blocking condenser to an intermediate point in the loading inductance of said first tuned circuit, the portion of the loading inductance thus included in the plate circuit being such that the coupling between the plate circuit and the loading inductance assists the magnetic coupling between the plate coil and the high frequency inductance.

12. A tuned interstage coupling system of the type including two tuned circuits in series between the adjacent tubes, said coupling comprising two substantially identical tuned circuits each having a tuning condenser and the common terminals of said condensers being grounded, a shunt coupling impedance connected to ground and to the joined terminals of loading inductances individual to said tuned circuits, a high frequency inductance completing each of said tuned circuits, a connection to the control grid of the second tube from the junction of the tuning condenser and high frequency inductance of said second tuned circuit, the plate circuit of the rst tube including a coil coupled magnetically to the high frequency inductance of said first tuned circuit, the low potential terminal of said coil being connected through a blocking condenser to an intermediate point in the loading inductance of said rst tuned circuit, the portion of the loading inductance thus included in the plate circuit being such that the coupling between the plate circuit and the loading inductance assists the magnetic coupling between the plate coil and the high frequency inductance, and means for shortcircuiting the loading inductance of said first tuned circuit and the portion thereof which is common to the plate circuit of said first tube.

13. The invention as set forth in claim l0, wherein said shunt coupling impedance is resonant at a frequency within the range to which said coupling system may be tuned.

14. The invention as set forth in claim 10, wherein said shunt coupling impedance is resonant at a frequency within the low frequency band to which said coupling system may be tuned.

15. The invention as set forth in claim 10, wherein said shunt coupling impedance is resonant at a frequency near the lower limit of the low frequency band to which said coupling system may be tuned.

16. An interstage coupling for use at radio frequencies comprising a pair of cylindrical forms, one of said forms being mounted at the end of and with its axis normal to the axis of the second form, a high frequency winding on said rst form, a coil within said first form and serving as a primary for said high frequency winding, a loading inductance wound on said second form and. connected to one terminal of said high frequency winding, a condenser and a radio frequency choke mounted within said second form, said choke being arranged to have substantially Zero magnetic coupling to said loadingwinding, a connection from said coil to said choke and to said condenser, and a. connection from the other terminal of said condenser to an intermediate point in said loading winding.

17. A radio-frequency coupling system adapted to operate over a plurality of frequency bands comprising a plurality of series-connected nonmagnetically coupled coils, a circuit including means individually coupled to each of said coils, and means for shortcircuiting one of said seriesconnected coils.

18. A radio-frequency coupling system adapted to operate over a plurality of frequency bands comprising a plurality of series-connected nonmagnetically coupled coils, a resistance connected across one of said coils, a circuit including means individually coupled to each of said coils, and means for shortcircuiting said last mentioned coil and its shunt resistance.

19.l A tuned radio-frequency coupling system adapted to operate over a plurality of frequency bands comprising a plurality of series-connected non-magnetically coupled coils and a variable condenser connected in shunt to said coils, a circuit including means individually coupled to each of said coils, and means for shortcircuiting one of said series-connected coils.

20. A tuned radio-frequency coupling system adapted to operate over a plurality of frequency bands comprising a plurality of series-connected non-magnetically coupled coils and a variable condenser connected in shunt to said coils, a resistance connected across one of said coils, a circuit including means individually coupled to each of said coils, and means for shortcircuiting said last mentioned coil and its shunt resistance.

21. In a radio receiver adapted to operate over a plurality of frequency bands, a plurality of tunable circuits each comprising a plurality of series-connected non-magnetically coupled coils and avariable condenser, a plurality of corresponding circuits, each including means individ- ,ually coupled to each of said coils, and unicon- A 'trol means for shortcircuiting the corresponding coils and a variable condenser, a plurality of corresponding circuits, each including means individually coupled to each of said coils, unicontrol means for shortcircuiting the corresponding coils of said tunable circuits, and unicontrol means for simultaneously adjusting the variab-le condensers of said circuits.

23. In a radio receiver adapted to operate over a plurality of frequency bands, a plurality of tunable circuits each comprising a plurality of series-connected non-magnetically coupled coils and a variable condenser, a plurality of corre'- sponding circuits, each including means individually coupled to each of said coils, unicontrol means for shortcircuiting the corresponding coils of said tunable circuits, and unicontrol means for simultaneously and similarly adjusting said tunable circuits to a common desired frequency.

24. In a radio receiver adapted to operate over a plurality of frequency bands, a plurality of tunable circuits each comprising a plurality of series-connected non-magnetically coupled coils and a variable condenser, a corresponding nurnber of primary circuits each having means individually coupled to each of the coils of one of the tunable circuits, unicontrol means for shortcircuiting the corresponding coils of said tunable circuits and for eliminating the primary circuit coupling means to said shortcircuited coils, and unicontrol means ior simultaneously adjusting the Variable condensers of said circuits.

25. A tuned radio-frequency coupling system connected between successive vacuum tubes in a radio-frequency amplier adapted to operate over a high-frequency band and over a low-frequency band, comprising series-connected, nonmagnetically coupled, high-frequency and lowfrequency coils connected to the input electrodes of each tube, said series-connected coils having coupled thereto the output circuit of the preceding tube, said output circuit having means individually coupled to each of said coils, and unicontrol means for shortcircuiting the low-frequency coils when it is desired to operate the system to receive signals in the high-frequency band.

26. A tuned radio-frequency coupling system connected between successive vacuum tubes in a radio-frequency amplier adapted to operate over a high-frequency band and over a low-frequency band, comprising series-connected, nonmagnetically coupled, high-frequency and lowfrequency coils connected to the input electrodes of each tube, a variable condenser connected to said high-frequency and low-frequency coils and forming therewith a tunable circuit, said seriesconnected coils having coupled thereto the output circuit of the preceding tube, said output circuit having means individually coupled to each of said coils, unicontrol means for simultaneously adjusting the successive tunable circuits to a common desired frequency, and unicontrol means for shortcircuiting the low-frequency coils when it is desired to operate the system to receive signals in the high-frequency band.

WILLIAM D. LOUGI-ILIN. CHRISTOPHER J. l'RtANKS.v 

